American teens are staying away from cigarettes in greater numbers than they have in generations. That’s among the good news in a new survey of more than 41,000 students published on Tuesday. But for the first time in a national survey, teens reported puffing on electronic cigarettes more than smoking conventional tobacco.

Teens are behaving better when it comes to drinking alcohol and abusing pills such as Vicodin and OxyContin. Cigarette smoking among high school students has been declining since the late 1990s, and the drop got sharper in the last few years. Today just 6.7 percent of high school seniors light up daily, less than half as many as a decade ago.

The survey, from the National Institute on Drug Abuse and the University of Michigan, jibes with other data showing less risky behavior from American teens. But one emerging risk has public health officials worried: electronic cigarettes, which the survey asked about for the first time this year. More teens are now using e-cigarettes than are smoking conventional cigarettes, with 17 percent of high school seniors reporting that they’d vaped in the last 30 days, compared with 13 percent who reported smoking.

It’s hard to say exactly what this means for the future health of Americans. E-cigarettes’ risks, especially for long-term users, are unknown, but they’re broadly believed to be less harmful than tobacco. And researchers are still trying to understand patterns of behavior. Are people who use e-cigarettes in adolescence likely to turn to tobacco later on? Are they more prone to abuse other drugs, because nicotine exposure affects the brain’s reaction to pleasure? Do e-cigarettes keep young people from adopting more harmful habits?

E-cigarette use has about doubled every year since 2011, according to Richard Miech of the University of Michigan, and the products are too new for experts to fully grasp the consequences. “The best we can do is follow people who have used e-cigarettes and see what happens to them as they age,” Miech says.

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